1. Technical Field
The invention relates to an apparatus and method for spin coating solutions onto the surface of a semiconductor wafer.
2. Background
The current practice in semiconductor manufacturing is to use thin film fabrication techniques. A large variety of materials can be deposited using thin films, including metals, semiconductors, insulators and the like. The composition and uniformity of these thin layers must be strictly controlled to facilitate etching of submicron features. Many of these materials are best suited to application of the substrate with a liquid which is then dried to form the solid thin film. The liquid materials are most often deposited using either spin or spray coating methods.
In a conventional spin coating process the semiconductor wafer to be processed is placed on a rotatable chuck and held in place by vacuum. The chuck is referred to by a variety of names, including spin chuck and vacuum chuck. The spin chuck has a diameter slightly less than that of the semiconductor wafer. The wafer is positioned on the chuck such that it is resting in a level horizontal plane with the inactive surface, designated as the bottom, in contact with the chuck and the opposite top surface is coated with the desired solution. In standard systems the chuck is powered and rotated by a motor.
In the spin coating process, the solution can be dispensed prior to spinning the wafer, which is referred to as static dispense or after the semiconductor wafer has been set in motion, which is termed dynamic dispense. In either case after the solution has been dispensed onto the top surface, the wafer is spun at a constant speed to establish a desired relatively uniform thickness of the solution across the wafer. Once the liquid layer acquires the relatively uniform and symmetrical profile, the remainder of the spin cycle allows the solvent in the solution to evaporate to produce a solid film on the wafer top surface.
The supply of the solution is dispensed onto the wafer from a supply nozzle. The nozzle can either be configured to simply drop a specific quantity onto the semiconductor wafer surface in the form of a puddle or to spray the desired quantity onto the wafer surface in the form of a mist.
After the solution is dispensed onto the wafer it is distributed uniformly over the surface largely as a result of the radial distribution of the liquid due to centrifugal and drag forces created by the spinning of the wafer.
The solution deposited on the wafer goes through a number of stages during the spin process, primarily due to the fluid dynamics created by the spinning substrate. At the start of the spinning a wave of solution is created that moves towards the edge of the wafer. As the major portion of the solution in the wave reaches the edge of the wafer it forms a ridge, this is referred to as the corona stage. A bead subsequently forms along the edge of the wafer as the solvent evaporates from the ridge formed in the corona stage. As the corona disappears the remaining solution leaves the surface in the form of a fine spiral-like mist. This spiral stage results in thousands of droplets spinning off the wafer and splashing back onto the wafer off of the surrounding spinner bowl. Bowls and splashguards have been designed to prevent this splashing. Additionally, solvent washing of the bottom of the wafer can eliminate the edge bead that forms during the spin coating.
Every layer deposited on the top surface of the wafer that possesses irregularities and variations in its uniformity has an adverse affect during all subsequent processing steps that the wafer undergoes. Uniformity of the layers is a critical factor in semiconductor wafer production. The film thickness uniformity obtained using the spin coating process is largely a function of size and shape of the wafer, because of the influence centrifugal force has in the spin coat process. The fluid dynamics described above become more pronounced as the diameter of the wafer used increases and the trend is towards using larger wafers.
To compensate for these undesirable influences the standard practice is to use a large starting volume of solution in the spin coating process. A large starting volume of solution also translates into a large amount of wasted solution. The increased amount of solution used also means an increase in the cost of production for semiconductor wafers. Approximately 30%-90% of the process solution used in the spin coating process is wasted in the form of excess solution that is thrown off of the wafer substrate. The excess solution is deposited to assure a thin uniform layer in the end product.
There have been a number of inventions proposed to alleviate these problems. U.S. Pat. No. 5,395,649 to Ikeda, employs a plate positioned above the wafer to change the air turbulence and fluid behavior on the wafer for improved layer uniformity.
In U.S. Pat. No. 5,405,813 to Rodrigues, a plurality of rotational speeds are used to increase layer uniformity and decrease the amount of starting solution required.
A number of patents use different types of nozzle mechanisms. U.S. Pat. No. 5,405,443 to Akimoto et al., discloses a nozzle that dispenses a fixed quantity of solution without entrainment of bubbles and particles utilizing a negative pressure system. U.S. Pat. No. 4,267,212 to Shinichi, includes moving the conventional spin coat nozzle across the radius of the wafer during solution application while rotating the wafer at a first and second speed. U.S. Pat. No. 5,403,617 to Haaland, enlists a computer controlled droplet generator to select droplet size and velocity to cause impact with the wafer without splashing. There is still a demand in the semiconductor wafer manufacturing industry for more economical means of solution application to the wafer that improves the uniformity of the process layers on the wafers and uses less chemicals.
The other major process used to deposit coatings on to wafer substrates is a spray coat process. The spray coating process permits much more efficient use of the process solution because the large starting excess needed for the spin coating process is not needed for the spray coating process. The problem with conventional spray coat processes are that they require thorough and comprehensive optimization of the process to obtain the quality of uniform layer thickness that is more easily obtained with the spin coat process. With the spray coat process the size and shape of the wafer have little effect on the end result. The uniformity of thickness of dielectric coating obtained using the spray process is determined by the sweeping motion of the spray nozzle.
An alternative to using either the spin coat or spray coat processes and their inherent problems is to coat the wafers using chemical vapor deposition (CVD). The CVD process includes the following basic steps: a) a known composition of reactant and inert gases is introduced into a reaction chamber; b) the gas species move to the substrate; c) the reactants are decomposed and chemically reacted at a heated surface of the substrate; e) the gaseous by-products are desorbed and removed from the reaction chamber. With the CVD process high purity films can be formed and deposited and a greater variety of starting compounds can be used. There are certain compositions that cannot be adequately applied to the wafer by any other process. The CVD process also had certain disadvantages. It increases both the cost of wafer production and increases the complexity of manufacturing the wafer. There are also often defects in the uniformity of the layers deposited on the wafer using the CVD process. Because of the increased cost and complexity of the CVD process it is still used far less that either spin or spray coating of the dielectrics onto semiconductor wafers.